This proposal examines the nature of circulating immunoreactive insulin in the insulin dependent diabetic. It is generally assumed that injected insulin is absorbed from the subcutaneous tissues intact. However, we have discovered an insulin-like substance, twice the size of insulin, in the circulation of nearly every diabetic tested to date in our laboratory. This insulin-like material (ILM) shares many of the characteristics of an insulin dimer and it may be formed in the subcutaneous tissues. The amount of ILM varies widely but can account for nearly 50% of the total insulin immunoreactivity detected in the blood of the type I diabetic. There seems to be more of this material in patients using beef-containing insulin and considerably less in those who use biosynthetic human insulin. It is tempting to hypothesize that the relative solubility of human insulin prevents the formation of this high molecular weight product. The specific aims of this proposal are to characterize this novel product of therapeutic insulin and determine its physiologic or pathophysiologic consequences on the insulin-requiring diabetic. There are reasons to be concerned that this material may have deleterious effects on the diabetic. First, insulin dimers and high molecular weight insulin products may act as potent antigens, immunizing the diabetic against insulin and resulting in the production of anti-insulin antibodies. Second, insulin dimers probably lack biologic activity and they may act as true competitive antagonists to insulin. This could cause or contribute to a state of insulin resistance and suboptimal diabetic control. Third, these "foreign" proteins may play a primary role in the production of diabetic end-organ complications. ILM will be measured in diabetics during treatment with various types of insulin given by the subcutaneous and intravenous routes. Hyperinsulinemic, euglycemic clamps will be performed in normal and diabetic volunteers to determine any in vivo role for ILM in insulin resistance. Biologic activity of ILM will be tested in vito using the isolated adipocyte method. A major effort will be directed towards characterizing the material and offering a structural determination. Finally, there will be a preliminary attempt to determine whether there is any relationship between ILM and end-organ damage in a retrospective fashion.